Former Empress Michiko has been in poor health since mid-September, vomiting multiple times and losing weight, with stress seen as one of the possible causes, the Imperial Household Agency said Friday.

The 85-year-old underwent breast cancer surgery in early September and has since been receiving hormonal treatment. The agency said the post-surgery treatment is not suspected to be the cause of her vomiting, which included a small amount of blood.

Related coverage:

Japan empress recovering, performs key official duties with emperor

Japan's former Empress Michiko turns 85, celebrations shelved over typhoon

Japan's former Empress Michiko leaves hospital after breast cancer surgery

She has also been found to have a cardiac valve abnormality and irregular pulse when she underwent a checkup in June over a rise in levels of heart failure markers. The levels remain high and doctors are monitoring her condition, according to the agency.

The former empress and her husband, former Emperor Akihito, have been receiving health checks twice a year and are scheduled to undergo this year's second round later this month. But the agency said doctors will conduct tests whenever necessary.

The couple retired as emperor and empress on April 30 after more than 30 years in the imperial positions. Their oldest son Emperor Naruhito and his wife Empress Masako succeeded them the day after.